1. Field of the Invention
The present invention is generally in the field of semiconductor fabrication. More specifically, the invention is in the field of reducing contaminants during semiconductor fabrication.
2. Background Art
In silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”) technology, SiGe HBTs are fabricated by techniques including, among other things, depositing doped silicon layers and undoped silicon layers on semiconductor dies, which occur in a reactor chamber. During deposition of doped silicon, dopants such as arsenic and phosphorous adhere to surfaces within the reactor chamber such as a susceptor, chamber sidewalls and the chamber's pre-heat ring.
HBT formation typically includes a “cleaning” process, which is performed after depositing doped silicon layers and before depositing undoped silicon layers, to reduce dopant concentration levels within a reactor chamber, which can contaminate undoped silicon layers during deposition. The cleaning process must reduce the concentration of dopants, i.e. contaminants, within a reactor chamber to less than 1×1016 atoms per cubic centimeter (i.e. 1×1016 cm−3) to prevent contamination of undoped silicon during the undoped silicon deposition process.
One conventional cleaning process, referred to as “etch-coat”, comprises etching the reactor chamber and thereafter coating the reactor chamber with undoped silicon. Disadvantageously, the etch-coat cleaning process cannot effectively reduce unwanted dopants when dopant concentration levels exceed 1×1017 cm−3. During HBT fabrication, dopant concentration levels can exceed 1×1020 cm−3. Thus, the etch-coat cleaning process is ineffective for some HBT fabrication processes.
Therefore, a need exists for reducing contaminants in a reactor chamber, which allows deposition of undoped semiconductors, such as undoped silicon, with reduced contamination from dopants left in the reactor chamber.